This invention relates to gas turbine engines and, more particularly, to the repair of stationary shrouds found in gas turbine engines.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The turbine blades are mounted on a turbine disk, which rotates on a shaft inside a generally cylindrical tunnel defined by a hollow stationary shroud structure. The stationary shroud structure is formed of a series of stationary shrouds that extend around the circumference of the tunnel in an end-to-end fashion. The stationary shroud structure has such a segmented arrangement to accommodate the thermal expansion experienced during each engine cycle as the stationary shroud structure is cycled between room temperature and a maximum service temperature of over 2000xc2x0 F. Each of the stationary shrouds has an internal gas path surface that is a segment of a cylinder, and a support structure that backs the gas path surface and provides for attachment to the adjacent structure.
During service, the support structure of the shrouds may be damaged by fatigue, erosion, and other mechanisms. One form of the damage is the wearing away of material from the shrouds, at locations such as the end faces, the forward and aft edges, and elsewhere. As material is worn away and during multiple repair cycles when material is removed by machining operations, the shroud gradually becomes undersize in at least one dimension of the support structure. When the shroud has become too small in at least one dimension of the support structure to continue to be functional, it is discarded.
There is a need for an improved approach to responding to such damage to gas turbine engine shrouds. The shrouds are made of expensive nickel-base or cobalt-base superalloys, and the discarding of a shroud represents a substantial cost. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method of repairing a gas turbine engine stationary shroud. The repair may be performed on any portions of the support structure. It is preferably performed on the end faces which butt against the end faces of the neighboring shrouds in service and gradually become undersized. The repaired shroud is fully functional and is serviceable at a small fraction of the cost of a new shroud.
A method of repairing a gas turbine engine stationary shroud comprises the steps of providing the gas turbine engine stationary shroud having an undersize repair region made of a shroud material, wherein the repair region is not located on a gas flow path surface of the gas turbine engine stationary shroud. The repair region may be, for example, an end face, an edge, or a back surface of the gas turbine engine stationary shroud. The repair region of the gas turbine engine stationary shroud is repaired so that the repair region is no longer undersize. The step of repairing includes the steps of providing a sufficient mass of a repair material comprising a first fraction of a first powder of a first alloy component, and a second fraction of a second powder of a second alloy component. The first alloy component and the second alloy component have different solidus temperatures. Each of the two powders is preferably prealloyed, so that its constituents are melted together prior to the two powder types being mixed together. The step of repairing further includes placing the repair material in the repair region, heating the repair material and the repair region to a brazing temperature sufficient to melt the repair material but not the shroud material of the repair region, so that the repair material flows over the repair region, and thereafter cooling the melted repair material and the repair region to solidify the repair material, the repair material having a solidus temperature less than that of the shroud material.
The shroud material may be a cobalt-base superalloy or a nickel-base superalloy, and the repair material is selected accordingly. The first powder and the second powder that form the repair material may be provided as free-flowing powders, or they may first be mixed and sintered together to form a pre-sintered compact. The use of the pre-sintered compact is preferred for standard repair locations, such as for use at the end faces.
The present approach achieves a fully serviceable repaired shroud, reducing the number of shrouds that are discarded. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.